This invention relates to the production of insulations of foamed plastic material, and refers in particular to a method and an apparatus for producing foamed plastic insulations, such as foamed polyurethane, in doors and cabinets for refrigerators, freezers and the like.
We shall refer hereinafter to the production of insulations of foamed polyurethane, however it is obvious that the invention can be likewise applied and adapted for producing foamed insulations in any type of article and with any suitable type of foamable plastic material.
In the manufacturing of cabinets and doors for refrigerators, freezers and the like, which make use of a rigid foam for the insulating material, the demolding time, that is to say, the length of time the cabinet or door remains in the press or foaming jig, greatly reduces the possibility of automating and improving the manufacturing process.
In the foaming of refrigerator cabinets, for example with polyurethane material, to which reference will be made hereunder, the demolding time, that is to say, the length of time each cabinet must remain in the jig or mold until the foam has cured or reaches the desired degree of hardness, depends upon several parameters or variables of the foaming process, which include the temperature of refrigerator cabinets and the foaming jig, the percent of overpacking, or excess of quantity of polyurethane mixture fed into the gaps or space between the outer and inner walls of the cabinet or in the door.
At present, in the manufacturing of refrigerator cabinets and the like, the average demolding time or length of time the cabinet remains in the foaming jig, for a model of cabinet, is either calculated by trial and error at the beginning of each manufacturing cycle or derived from previous productions, with complicated operations which involve losses in terms of manhours and manufacturing rejects. In practice, at the start of production, trial foaming operations are carried out on a number of refrigerator cabinets, varying both the length of time in which the cabinet remains in the jig and the percent of additional charge or overpacking of mixture required in order to ensure that it is properly filled, until the foaming conditions considered suitable for that particular cabinet or article to be manufactured are reached.
According to the known manufacturing systems, the cabinets are demolded from the foaming jigs after a pre-established period of time, which remains constant throughout the entire production period; however, as the foaming conditions can vary during manufacturing, as a rule the length of time that the cabinets remain in the jigs is suitably extended so as to have a margin of safety in order to eliminate or at least reduce any possible problems to a minimum.
This results in an excessively low productivity due to the considerable length of time that cabinets remain in the foaming jigs, and insufficient utilization of the productive capacities of the plants. This mode of operating proves to be impractical and gives rise to manufacturing rejects and an excessive consumption of material; furthermore, it does not ensure the manufacturing of insulations or articles with constant uniform properties, due to the fact that certain parameters of the foaming process, in particular, the pressure exerted by the gases in the polyurethane foam, change during the production cycle. Consequently, it becomes necessary to carry out additional tests, sometimes operating with excess of overpacking and/or leaving the cabinet in the jig for excessively long periods, with respect to what would be necessary in actual fact.
In an article by W. D. Clarke in Polyurethane Marketing & Technology--SPI 28 annual conference--November 1984, pages 194 and 197, a laboratory method was proposed as an experiment in order to study the variable which affect the foaming process, considering the changes with time in the pressure of the foam inside a suitably modified Brett mold. In substance, W .D. Clarke maintained that he was able to calculate the demolding time by detecting the pressure curves of the polyols and comparing the pressure drops which occur after a given period of time has elapsed, with respect to a specimen curve, starting from the peak pressure.
Although W. D. Clarke proposes a simpler system for calculating the length of time the cabinet should remain in the foaming jig, in industrial use this does not give rise to any substantial difference due to the fact that the refrigerator cabinets will still have to be demolded after a pre-established and suitably prolonged period of time in order to make allowance for any possible changes in the parameters of the foaming process which will occur during manufacture, without any possibility of controlling the foaming process or of intervening automatically in order to vary a subsequent foaming operation.
The scope of this invention therefore is to provide a method for producing foamed plastic insulations in doors and cabinets of refrigerators, freezers and the like, by means of which it is possible to automatically control the foaming of a door or a cabinet closed in its respective foaming jig, and at the same time to establish, for each individual foaming operation, the most appropriate moment for demolding the foamed article, determining such moment each time no longer in relation to a preestablished length of time for the cabinet or door to remain in the foaming jig, but in relation to one or more suitably controlled characteristic parameters of the foaming process.
A further scope of this invention is to provide a method as defined above, which enable the total automation of the operations of foaming and removing the foamed article from the jig, making it possible to intervene from time to time and to automatically modify certain parameters of the process, for example the percent of overpacking of mixture, if necessary, in order to maintain the most appropriate foaming conditions, without having to carry out additional tests.
A still further scope of this invention is to provide a method for foaming doors and cabinets of refrigerators, freezers and the like, with which it is possible to obtain a production of insulating material with substantially uniform and constant characteristics and which at the same time reduce the length of time that each single cabinet or each single door remains in the foaming jig, thereby ensuring higher productivity, greater utilization of the plants and immediate quality control.